1. 1 CSC 533: Organization of Programming Languages Spring 2005 OOP in Java  object-oriented programming  inheritance & polymorphism  interfaces  abstract classes  Java vs. JavaScript

2. 2 OOP in Java recall: Java classes look similar to C++ classes public class Person { public Person(String nm, String id, char sex, int yrs) { name = nm; ssn = id; gender = sex; age = yrs; } public void Birthday() { age++; } public void Display() { System.out.println("Name: " + name + "\nSSN :" + ssn + "\nGender: " + gender + "\nAge: " + age); } private String name, ssn; private char gender; private int age; } class Person { public: Person(string nm, string id, char sex, int yrs) { name = nm; ssn = id; gender = sex; age = yrs; } void Birthday() { age++; } void Display() { cout << "Name: " << name << endl << "SSN : << ssn << endl << "Gender: " << gender << endl << "Age: " << age << endl; } private: string name, ssn; char gender; int age; };

3. 3 OOP in Java (cont.) inheritance in Java looks similar to C++  recall: all reference types are automatically derived from the Object class class Student : public Person { public: Student(string nm, string id, char sex, int yrs, string sch, int lvl) : Person(nm, id, sex, yrs) { school = sch; grade = lvl; } void Advance() { grade++; } void Display() { Person::Display(); cout << "School: " << school << endl << "Grade: " << grade << endl; } private: string school; int grade; }; public class Student extends Person { public Student(String nm, String id, char sex, int yrs, String sch, int lvl) { super(nm, id, sex, yrs); school = sch; grade = lvl; } public void Advance() { grade++; } private String school; private int grade; } super calls the constructor of the parent class

4. 4 OOP in Java (cont.) similar to C++, can override existing methods  specify methods of the parent class using super class Student extends Person { public Student(String nm, String id, char sex, int yrs, String sch, int lvl) { super(nm, id, sex, yrs); school = sch; grade = lvl; } public void Advance() { grade++; } public void Display() { super.Display(); System.out.println("School: " + school + "\nGrade: " + grade); } private String school; private int grade; }; all (non-private) method calls are dynamically bound in Java  don’t need to specify virtual

5. 5 OOP in Java (cont.) ArrayList people = new ArrayList (); Person p = new Person("Bjarne", "123-45-6789", 'M', 20); people.add(p); Student s = new Student("Grace", "9876-54-321", 'F', 21, "Creighton", 16); people.add(s);... for (int i = 0; i < people.size(); i++) { people.get(i).Display(); } recall: in C++, could create a structure that mixed parent & derived objects  needed to utilize pointers to dynamically allocated objects in Java, all library data structures store reference types  thus, can automatically mix objects  since methods are bound dynamically, Display() calls the appropriate method

6. 6 Advanced OOP in Java in Java, can disallow inheritance explicitly  when a method is declared final, it can't be overridden public final void Birthday() { age++; }  when a class is declared final, it can't be inherited from (e.g., Math, Integer) no multiple inheritance in Java, but can have multiple interfaces  an interface describes the methods required for a derived class public interface List { boolean add(T o); void add(int index, T o); boolean contains(T o); T get(int index); boolean isEmpty(); int size();... }

7. 7 List interface interfaces are useful for grouping generic classes  a derived classes implements an interface  can have more than one implementation, with different characteristics public class ArrayList implements List { private T[] items;... } public class LinkedList implements List { private T front; private T back;... }  using the interface, can write generic code that works on any implementation public numOccur(List words, String desired) { int count = 0; for (int i = 0; i < words.size(); i++) { if (desired.equals(words.get(i))) { count++; }

8. 8 Comparable interface another useful predefined interface is Comparable public interface Comparable { int compareTo(T other) }  any class of objects that can be compared should implement Comparable public final class Integer implements Comparable { … } public final class String implements Comparable { … } public class Name implements Comparable { … } the Collections utility class contains a variety of methods that work on Lists  e.g., copy, fill, frequency, reverse, shuffle, …  if elements are Comparable, then additional methods e.g., sort, binarySearch, min, max, …

9. 9 Interface example: HW3-4 public interface Statement { void Read(Tokenizer program); void Execute(VarTable vars); String GetType(); String toString(); } public class AssignStatement implements Statement { public AssignStatement() {... } public void Read(Tokenizer program) {... } public void Execute(VarTable vars) {... } public String GetType() {... } public String toString() {... } private String lhs; private Expression rhs; } public class OutputStatement implements Statement { public OutputStatement() {... } public void Read(Tokenizer program) {... } public void Execute(VarTable vars) {... } public String GetType() {... } public String toString() {... } private String constant; private Expression rhs; } can define an interface that specifies what any kind of statement must do  in Java, enable output by defining a method that converts an Object to a string – toString method is automatically called when outputting have multiple classes implement the Statement interface  if derived class fails to implement any method from the interface, then compiler will complain

10. 10 Abstract class example: HW3-4 public abstract class Statement { public abstract void Read(Tokenizer program); public abstract void Execute(VarTable vars); public abstract String GetType(); public abstract String toString(); public static Statement GetNext(Tokenizer program) {... } } public class AssignStatement extends Statement { public AssignStatement() {... } public void Read(Tokenizer program) {... } public void Execute(VarTable vars) {... } public String GetType() {... } public String toString() {... } private String lhs; private Expression rhs; } public class OutputStatement extends Statement { public OutputStatement() {... } public void Read(Tokenizer program) {... } public void Execute(VarTable vars) {... } public String GetType() {... } public String toString() {... } private String constant; private Expression rhs; } Note: an interface does not have any code to be inherited  so multiple interfaces are OK if you want to have derived classes inherit some common code, but still ensure methods  define an abstract class  derived class must implement the abstract methods in order to compile

11. 11 Java vs. JavaScript interesting to consider a third C++ variant: JavaScript  designed to be a scripting language for execution in and enhancements of a Web browser  developed at Netscape in 1995, integrated into Navigator later adopted by IE under the name JScript (some variations)  as with Java, chose to keep basic syntax of C++ to aid learning  different design goals yield different features recall: Java took many features from C++, but removed/added features due to different design goals e.g., platform independence  interpreted+compiled execution model ease of development over efficiency  dynamic binding, garbage collection simplicity over expressivity  no goto, no implicit coercions, no operator overload security  no pointer access, byte code verifier

12. 12 JavaScript design intended to be a scripting language for Web pages  JavaScript code is embedded directly into HTML, interpreted by browser  a.k.a., client-side scripting scripting applications are more quick-and-dirty, relatively small  variables are bound to type and address dynamically for flexibility  do not need to declare variables, functions are not typed either  code size is limited by the browser not expected to develop large applications  object-based: lots of useful classes predefined (Array, String, Math, …) can define new classes but awkward, no info hiding, no inheritance user security is important, script code security isn't  like Java, JavaScript code can't access local files  no way to hide the JavaScript source when download Web page

13. 13 JavaScript example Dave's Hello World Page function SayHello(name) { if (name == "Dave") { document.write("It's an honor, Dave!"); } else { for (var i = 0; i < 10; i++) { document.write("Hello " + name + ", glad to meet you! "); } userName = prompt("Enter your name:", "Your name"); SayHello(userName); JavaScript code can be embedded in the page using SCRIPT tags code is executed by the browser, output is inserted into the page document.write is JavaScript's output routine control structures are similar to C++/Java no type for function, parameters can declare local variables

14. 14 JavaScript objects // Die.js function Die(sides) { this.numSides = sides; this.numRolls = 0; this.Roll = function () { this.numRolls++; return Math.floor(Math.random()*this.numSides)+1; } function ColoredDie(sides, color) { this.dieColor = color; this.Die(sides); } ColoredDie.prototype = new Die; ColoredDie.prototype.Die = Die; many useful objects w/ methods are predefined e.g., Math.abs document.lastModified can define new classes can even utilize inheritance very awkward syntax no info hiding can add data/methods dynamically die1.owner = "Dave" also, can put useful code in a separate file, load using SCRIPT tag with SRC attribute Roll two dice die1 = new Die(6); die2 = new ColoredDie(6, "blue"); roll1 = die1.Roll(); roll2 = die2.Roll(); document.write("I rolled " + roll1 + " and " + roll2 + " ==> " + (roll1+roll2) + " ");

15. 15 JavaScript form processing Dice Stats function DoIt() { var die1 = new Die(6); var die2 = new Die(6); document.images.die1.src = "Images/die"+die1.Roll()+".gif"; document.images.die2.src = "Images/die"+die2.Roll()+".gif"; document.DiceForm.numRolls.value = parseFloat(document.DiceForm.numRolls.value) + 1; } Number of rolls: main uses of JavaScript processing form elements (e.g., text box) reacting to user- controlled events (e.g., button click)

16. 16 JavaScript vs. Java implementations consider a text-manipulation example: convert English text to pirate talk Java application  divided into 2 classes: Translator.java Translator.java : a generic language translator PirateTalk.java PirateTalk.java : GUI front-end, displays label & text area & button  translation vocabulary is read in from a file, so easily changed JavaScript program (i.e., interactive Web page)  all contained in a single Web page PirateTalk.html  since can't read external files, vocabulary must be embedded in the page  GUI elements are much simpler (since HTML), easy to embed images